For industry, a method for producing a pure-oxygen gas is mainly a cryogenic method of cryogenic rectification of liquefied air, which has the drawbacks of huge equipment size and process complexity, and is only suitable for large-scale industrial production. Although molecular sieve pressure swing adsorption (PSA) method is suitable for indoor fields, the purity of the obtained oxygen is not higher than 95%. In the water electrolysis process represented by solid electrolyte water electrolysis technology, pure oxygen and pure hydrogen are produced at anode and cathode of a water electrolyzer at the same time and their purities can reach up to 99.99%, respectively. An operating solid electrolyte water electrolyzer has high current density (1-3 A/cm2) with above 85% of energy efficiency, and is safe, reliable and durable (only pure water is needed and electrolyte solution not required), and of compact structure, small volume and light weight. However, the power consumption for producing a pure-oxygen gas by solid electrolyte water electrolysis is relatively high, i.e. 8-10 kWH per cubic meter of oxygen. In addition, if the hydrogen produced at cathode of the water electrolyzer is not utilized, the hydrogen energy is not only wasted, but also the potential hydrogen safety issues related to its easy combustion and explosion do exist.
With respect to above-mentioned issues related to hydrogen safety, in China patents (Nos. 02114162.2 and 201110105892.8), water electrolyzers and hydrogen fuel cells are combined. Hydrogen generated by water electrolyzers flows in the hydrogen fuel cells to generate power. In the China patent No. 02114162.2, a dual power supply consisting of hydrogen fuel cells and one direct current (DC) supply by the rectification of an externally-connected alternating current (AC) of 220 V is used, which provide power to the water electrolyzers simultaneously. In a dual power supply, if the voltage of the externally-connected power supply is higher than the voltage of the fuel cells under operation, the fuel cells would be damaged by reverse charging. Due to the factors such as the load instability of the water electrolyzer, etc., in practical use, it is difficult to guarantee that the dual power supply can run stably, reliably and efficiently. In addition, because the electric energy collected by the hydrogen fuel cells only takes up about 30-40% of the consumed electric energy of the water electrolyzer, namely, 5-7 kWH per cubic meter of oxygen is still required, hence the operation cost for producing a pure-oxygen gas is still relatively high. Moreover, this patent is only suitable for the sites where the AC of 220 V is accessible. In the China patent No. 201110105892.8, to solve the problems of the dual power supply and the sites limitation (by accessibility of external AC power), a secondary battery is used to supply power to the water electrolyzer, and the secondary battery is charged by the hydrogen fuel cells in a floating charging way. However, because of being inhibited by the power capacity of a secondary battery and impeded by the problem of relatively high energy cost of water electrolysis, the limited amount of electric energy stored in the secondary battery will be quickly consumed during operation, so that the secondary battery must be recharged by an external power supply after operation for a while and restart to work. A device for producing an oxygen gas by this patent is only suitable for an intermittent use at low flow rate and of short time.